Polycarbonates are well-known as excellent materials for optical applications because of their inherent toughness and clarity. The most familiar linear polycarbonates are homopolymers derived from 2,2-bis(4-hydroxyphenyl)propane, commonly known as bisphenol-A (hereinafter, BPA). These materials are transparent and exhibit excellent thermal and mechanical properties. One application for polycarbonates is the fabrication of optical materials such as lenses and substrates for optical storage media.
During manufacturing, the polymeric polycarbonate is typically molded at high temperatures and pressures which, upon cooling, may lead to molecular orientations and stresses that are frozen into the material. In such cases, the cooled polycarbonate becomes anisotropic and exhibits orientational birefringence. As a light ray passes through a birefringent material, it is split into two plane-polarized light rays, each having a plane of polarization extending in a perpendicular direction relative to the other. Each light ray has a different index of refraction in the polymer, and the difference between these indices of refraction is referred to as the birefringence of a material. Because light passing through a birefringent material follows more than one path, distortion of the light results. Thus, birefringence is an undesirable property for polymers used in optical applications. Ideally, materials used in optical applications should have a birefringence substantially equal to zero.
Wimberger Friedl et al. reported in U.S. Pat. No. 5,424,389 and European Patent Application 0621297A2 that random copolycarbonates of BPA and 6,6'-dihydroxy-3,3,3',3'-tetramethyl-1,1'-spirobiindane (hereinafter, SBI), wherein the mole fraction of SBI varies from 0.844 to 0.887, show dramatic improvements in birefringence over BPA polycarbonate homopolymers, as measured by the stress-optical coefficient (C.sub.m). The stress-optical coefficient (C.sub.m) for a polymer is a measure of its sensitivity to orientational birefringence. Preferably, the absolute value of C.sub.m in polymers used in optical applications is substantially equal to zero. Likewise, Faler et al. disclosed in U.S. Pat. No. 4,950,731 that random SBI/BPA copolymers demonstrate improved optical properties as compared with BPA polycarbonates.
Although commercially available polycarbonate resins based solely on BPA exhibit excellent optical and mechanical properties, they are unsuitable for high temperature applications or further high temperature surface processing because the glass transition temperature (T.sub.g) value for BPA polycarbonates is relatively low, approximately 150.degree. C. Materials molded from BPA polycarbonates cannot withstand post-molding processing at temperatures greater than 150.degree. C., such as the application of chemically resistant hard coats or thick surface coatings often used in optical applications. Thus, a higher T.sub.g is critical for molded resins that undergo additional high temperature processing to maintain the integrity of the molded part.
SBI homopolycarbonates exhibit a high T.sub.g (up to 230.degree. C.), as disclosed in the aforementioned patent to Faler et al., but the mechanical strength and ductility of SBI materials are much reduced relative to the BPA polycarbonates. However, by adding and varying the amount of BPA monomer in spirobiindane (SBI) based polycarbonates, Faler et al. reported that the low T.sub.g of BPA polycarbonates can be counteracted. These SBI and BPA monomers used in combination produce copolymers with a higher T.sub.g but do not always retain the requisite toughness.
The present invention is based on the unexpected discovery that linear homopolymers derived from various indane bisphenols, as shown in the following structure (I), which are similar in structure to SBI homopolymers, demonstrate improved thermal behavior relative to BPA polycarbonates and improved ductility relative to SBI homopolycarbonates. The improvement in thermal properties and ductility is also observed in linear copolymers having a combination of the repeat units of structure (I) and the following structure (II). In addition, the clear indane homopolycarbonates and copolycarbonates bearing this combination show improved optical properties.